As and since

As and since can both be used to refer to the reason for something. They are used in the same way.

As he wasn't ready, we went without him.

Since she wanted to pass her exam, she decided to study well.

Since- and as-clauses cannot stand alone.

Because

Becauseoften introduces new information which is not known to the listener/reader. It puts more emphasis on the reason. When the reason is the most important part of the sentence, the because-clause usually comes at the end.

We had dinner after ten o' clock because dad arrived late.

He bought a new home because he won a lottery.

I read because I like reading.

A because-clause can stand alone.

Why are you crying? Because John hit me.

For

We use a for-clause when we introduce new information. A for-clause often expresses an inference. It cannot come at the beginning. It cannot stand alone either.

I decided to consult a doctor for I was feeling bad.

Something certainly fell ill; for I heard a splash.

All precautions must have been neglected, for the epidemic spread rapidly.